Physics (9203) | Sound and ultrasound

WAVES: Sound and Ultrasound

Welcome to the exciting world of sound! This chapter connects everything we’ve learned about waves—like frequency, wavelength, and speed—to the sounds we hear every day. Understanding sound waves is crucial not only for appreciating music but also for understanding incredible technologies like medical scans and depth finding. Don't worry if wave concepts seemed tricky before; we'll break down sound step-by-step!

1. The Nature of Sound Waves

Sound is essentially energy traveling through a substance. But how does it move?

1.1 How Sound is Produced

All sounds originate from vibrations.

• When you speak, your vocal cords vibrate.
• When you hit a drum, the skin vibrates.
• These vibrations push the particles in the surrounding medium (like air) back and forth, transferring energy.

1.2 Sound Requires a Medium

One of the most important facts about sound is that it must travel through a substance, or medium (a solid, liquid, or gas).

• Sound cannot travel through a vacuum (empty space).
• Analogy: Imagine a line of people holding hands. To pass a message, you squeeze the hand of the person next to you. If there’s nobody there (a vacuum), the energy stops! This is why explosions in space, as depicted in movies, would actually be silent.

1.3 Sound is a Longitudinal Wave

In our general waves chapter, we learned about two types of waves: transverse and longitudinal. Sound belongs to the latter.

A sound wave is a longitudinal wave.

This means the vibrations (the particle movement) are parallel to the direction the wave energy is traveling.

• Compression: The regions where the particles are pushed together and the pressure is highest.
• Rarefaction: The regions where the particles are spread apart and the pressure is lowest.

Memory Trick: Think of a Slinky spring pushed forward. The crowded coils are Compressions, and the stretched-out coils are Rarefactions.

2. The Speed of Sound

The speed at which sound travels depends entirely on the medium it is moving through.

2.1 Speed in Different Media

The closer the particles are packed and the stronger their connections, the faster sound can be transmitted.

1. Solids: Fastest speed (particles are tightly packed). Example: Steel, about 5,000 m/s.
2. Liquids: Medium speed. Example: Water, about 1,500 m/s.
3. Gases (Air): Slowest speed (particles are far apart). Example: Air at room temperature, typically around 330–340 m/s.

Key Concept: Sound travels significantly slower than light. This is why you see lightning flash instantly, but the thunder rumble takes a few seconds to reach you.

2.2 Calculating Wave Speed

The speed of sound, just like any wave speed, is calculated using the standard wave equation. Don't forget that frequency (f) is measured in Hertz (Hz) and wavelength (\(\lambda\)) is measured in meters (m).

$$v = f\lambda$$

Where:
\(v\) = speed (m/s)
\(f\) = frequency (Hz)
\(\lambda\) = wavelength (m)

Did you know? The speed of sound in air increases slightly if the temperature rises because the particles are moving faster.

3. Reflection of Sound: Echoes

When sound waves hit a hard, smooth surface, they can bounce back. This is called reflection, and the reflected sound wave is known as an echo.

3.1 How to Calculate Distance Using Echoes

Echo calculations are common in exams. The key concept is remembering that the sound must travel to the wall and back again to be heard as an echo.

Process Step-by-Step:

1. Measure the time taken (\(t\)) for the sound to travel to the barrier and return.

2. Use the formula: Distance = Speed × Time (\(D = v \times t\)) to find the total distance traveled.

3. To find the distance (\(d\)) from the source to the barrier, you must divide the total distance by two.

$$d = \frac{v \times t}{2}$$

Example: If the speed of sound is 340 m/s and an echo is heard after 0.5 seconds, the total distance traveled by the sound is \(340 \text{ m/s} \times 0.5 \text{ s} = 170 \text{ m}\). The distance to the wall is \(170 \text{ m} / 2 = 85 \text{ m}\).

Common Mistake to Avoid: Forgetting to divide the total distance (\(v \times t\)) by 2! Always remember the sound has to make the return journey.

4. Hearing, Frequency, and Ultrasound

The frequency of a sound wave determines its pitch. High frequency means high pitch (like a whistle); low frequency means low pitch (like a bass drum).

4.1 The Range of Human Hearing

Humans can only hear sounds within a specific range of frequencies.

• The lowest frequency a typical young person can hear is about 20 Hz.
• The highest frequency is about 20,000 Hz (or 20 kHz).

This range decreases as people get older, especially the high-frequency limit.

4.2 Defining Ultrasound

Any sound wave with a frequency above 20,000 Hz (20 kHz) is classified as ultrasound.

• Ultrasound is sound that humans cannot hear.
• Animals like bats and dolphins use ultrasound for navigation (echolocation).

Note: Sounds below 20 Hz are called *infrasound*, but the focus of the curriculum is primarily on ultrasound and its uses.

5. Applications of Ultrasound

Ultrasound waves are essential tools in science and technology because high-frequency waves (low wavelength) can be highly directional and produce detailed reflections.

5.1 Medical Applications (Scanning)

Ultrasound is widely used in medicine because it is non-ionising (unlike X-rays), meaning it does not carry enough energy to damage cells.

• Prenatal Scanning: Used to produce images of a foetus inside the womb.
• Internal Imaging: Used to view internal organs and blood flow.

The principle relies on timing the reflections (echoes) from boundaries between different tissues (like muscle and bone) to build up a picture.

5.2 Sonar and Depth Finding

Sonar (SOund Navigation And Ranging) uses ultrasound to measure the depth of the sea or locate underwater objects (like submarines or shipwrecks).

• A transducer on the ship emits a pulse of ultrasound.
• The pulse travels to the seabed and reflects back.
• By measuring the time (\(t\)) taken for the echo to return and knowing the speed of sound (\(v\)) in water, the depth (\(d\)) can be calculated using the echo formula: \(d = \frac{v \times t}{2}\).

5.3 Industrial Applications

Ultrasound is also used industrially to check the integrity of materials.

• Flaw Detection: Ultrasound pulses can be sent into metal components or pipes. If there is an internal crack or flaw, a sudden reflection (echo) will be received much earlier than expected, indicating the defect's location.